1. Field of the Invention
Synthetic polymeric membranes are used for separation of species by dialysis, electrodialysis, ultrafiltration, cross flow filtration, reverse osmosis and other similar techniques.
2. Description of the Prior Art
Prior art methods for forming synthetic polymeric membranes are discussed in Australian patent specification No. 505,494 of Unisearch Limited.
One such method which was developed by S. Loeb and S. Sourirajan at the University of California, Los Angeles concerned the formation of a membrane from a solution of a polymer (such as cellulose acetate) in a solvent (such as acetone media). A thin layer of the solution was spread onto a glass plate and the solvent allowed to evaporate. As the surface layer dried faster than the layer underneath, a skin surface layer was formed and beneath that was a semi-solid matrix. The membrane was then quenched in another solvent whereby the remaining polymer was precipitated to form a sponge-like backing of the membrane.
The membrane forming technique disclosed in Unisearch Australian patent specification No. 505,494 is broadly described as being the controlled unidirectional coagulation of the polymeric material from a solution which is coated onto a suitable inert surface. The first step in the process is the preparation of a "dope" by dissolution of a polymer. This is said to be achieved by cutting the hydrogen bonds (which link the molecular chains of the polymer together) with a solvent. After a period of maturation, the dope is then cast onto a glass plate and coagulated by immediate immersion in a coagulation bath which is capable of diluting the solvent and annealing the depolymerization compound which has been used. According to the one example given in the specification, the "dope" consisted of a polyamide dissolved in a solvent which comprised hydrochloric acid and ethanol.
In contradistinction to the polymeric solution teachings of Australian patent specification No. 505,494 and its discussion on the prior art, the present invention is based on the concept that the liquid material out of which the membrane is cast is a colloidal suspension so that the surface pore density is significantly increased over the surface pore density of prior membranes.